test-template-engine: do not evaluate unused keywords by custom engine

If the custom engine, "mytemplater", were installed as the default, it would
enter to an infinite recursion at stringify(v) because template keywords may
generate a nested mapping containing the same keywords.

Spotted by a future patch which will replace context.resource('templ')(...)
with context.process(...).

tests/test-template-engine.t

@@ -16,8 +16,7 b''
   >         props = self._defaults.copy()
   >         props.update(map)
   >         for k, v in props.items():
-  >             if k in (b'templ', b'ctx', b'repo', b'revcache', b'cache',
-  >                      b'troubles'):
+  >             if b'{{%s}}' % k not in tmpl:
   >                 continue
   >             if callable(v) and getattr(v, '_requires', None) is None:
   >                 props = self._resources.copy()